OK, I woke-up this morning find my very first "System Update" message on my 3 month-old S3 (on version LJ7). It's rooted with stock ROM, so I came here right away.
After a lot of reading and research, I changed first my recovery from CWM over to TWRP (which I have to say is pretty nice). Then I installed GooManager so I could update TWRP to the latest version of 2.4.3.0. No problem. (Hint: To update using GooManager, Install the app and open it. Tap menu then hit Install OpenRecoveryScript. Tap Yes. Verify that the filename displays your device's code name and hit Yes. The file will download and your device will install the recovery automatically.)
After all that was said and done, I booted to TWRP. That's when I ran into problems which took me a few hours to resolve.
Once I cleared the cache and dalvik (I did not perform a factory reset because I didn't want to go through the pains of setting everything back-up), I installed the 21MB baseband firmware file "Galaxy-S-fre3-MA6FirmwareModemAIO.zip" using TWRP with no issues.
However, when it came to installing the ROM, using the method originally posted by Ytsejamer and the 158MB "update_SPH-L710_LJ7_to_MA6.zip" file, I kept getting the same error at the end of the update process that led to "Installation Aborted" -- "Status 7".
Solution: Use the "MA6 Full Odexed Version" link Skunk provided with the 770MB "MA6_update_FULL_Odex.zip" file. That worked for me.
Now onto the MB1 update...